While, conventionally, feedback in lasers can be produced by a Fabry-Perot cavity, there has been strong recent interest in lasers in which feedback is produced by distributed scattering by a grating which provides for spatially periodic variation in effective index of refraction. Such so-called distributed feedback lasers (DFB lasers) are described in U.S. Pat. No. 3,760,292, issued Sept. 18, 1973 to H. W. Kogelnik et al., in the paper by H. Kogelnik et al., "Stimulated Emission in a Periodic Structure", Applied Physics Letters, Vol. 18 (1971), pp. 152-154, and in numerous further items in the open literature. As compared with conventional lasers, distributed feedback lasers have fewer allowed modes.
One aspect of distributed feedback laser design which receives particular attention in the following is a concern with mode stability or selectivity, i.e., the degree of insensitivity of single-mode laser operation with respect to unavoidable variations in operating conditions such as, e.g., variations in operating temperature or pump current. In this respect it has been found that the introduction of a central coupling-strength discontinuity promotes oscillation of a single mode in preference to the nearest redundant modes. Resulting preferred grating structure is described in U.S. Pat. No. 4,096,446, issued June 20, 1978 to H. A. Haus et al., and in the paper by H. A. Haus et al., "Antisymmetric Taper of Distributed Feedback Lasers", IEEE Journal of Quantum Electronics, Vol. QE-12 (1976), pp. 532-539.
Various methods have been proposed for the manufacture of gratings with a central discontinuity, and the effects of such a discontinuity have received a considerable amount of experimental as well as theoretical attention. The following items are cited as representative in these respects:
K. Sekartedjo et al., "1.5-.mu.m Phase-shifted DFB Lasers for Single-mode Operation", Electronics Letters, Vol. 20 (1984), pp. 80-81;
K. Utaka et al., "Analysis of Quarter-wave-shifted DFB Laser", Electronics Letters, Vol. 20 (1984), pp. 326-327;
F. Koyama et al., "1.5-.mu.m Phase-adjusted Active Distributed Reflector Laser for Complete Dynamic Single-mode Operation", Electronics Letters, Vol. 20 (1984), pp. 392-393; and
K. Utaka et al., ".lambda./4-Shifted InGaAsP/InP DFB Lasers by Simultaneous Holographic Exposure of Positive and Negative Photoresists", Electronics Letters, Vol. 20 (1984), pp. 1008-1010.
Common to grating structures disclosed in items as cited above is central or near-central placement of a discontinuity in a grating structure, the possibility of off-center placement having been either ignored or else considered as detrimental.